A passenger car tire is provided with a tread radially on the outside surmounting a crown belt itself placed radially on the outside of a reinforcement. This tread provides contact between the tire as it runs and the road and needs to allow the vehicle to be steered and stopped irrespective of the driving conditions. In rainy weather, the standing water on the road surface has to be able to be removed so that the tread can effectively remain in contact with the road. If this water cannot be removed, then upwards of certain driving speeds, a mechanism whereby the tire is lifted off the road (the aquaplaning phenomenon) occurs and is detrimental to roadholding and therefore to safety. To counter that, it is known practice to provide cavities and grooves in the tread of the tire to guide the water present into these cavities and grooves and discharge the water in the grooves to the outside of the contact surface both laterally and longitudinally.
Laterally or transversely here means a direction which is parallel to the axis of rotation of the tire.
Longitudinally (or circumferentially) on the tread here means the direction perpendicular to the transverse direction and tangential to a circle centered on the axis of rotation.
A transverse groove means a groove opening at one of its ends axially towards the outside of the tire to encourage the liquid to flow laterally with respect to the path followed by a tire as it runs along a road covered with water. For preference, such a groove is oriented in a direction that makes an angle smaller than 45 degrees with the axis of rotation of the tire.
In significant depths (for example at least 2 mm) of standing water and when the vehicle speed is high, it has been found that the flow mechanism can be disturbed and lead to reduction in performance particularly in terms of safety as the onset of the aquaplaning phenomenon notably manifests itself in a loss of contact with the road surface.
Document FR2152907 discloses a tire which, in addition to the longitudinal grooves, has radial cavities in the tread, these radial cavities, which are formed on the axially external parts of the tread, being extended by a cavity formed on the inside of the tread and emerging on the external lateral faces of the tread. The tire described in that document, thanks to these radial cavities, has additional capability to remove the water on the road. However, it has been found that the tire needs to be further improved for running on a road covered with water in order to provide even better roadholding.